This invention relates to a new process for the production of foam plastics based on polyisocyanates which have a top layer which is resistant to light and yellowing. A completely satisfactory commercial process for producing polyisocyanate foam plastics with a top layer which is resistant to light and yellowing has not, to date, been attained.
The principle which has been practiced for many years of producing such articles by a two-stage process in which a foam core is first produced in a mold and then covered or lacquered with a top layer of various materials such as textile fabrics, leather, PVC or polyurethane is very labor intensive and expensive. This is especially true if the foam cores have an irregular shape.
The other principle which has also been known for many years, of producing so-called integral foams, as described, for example, in U.S. Pat. No. 3,824,199; 3,726,952; 4,033,912; 4,024,090; 4,058,492 and 4,020,001, has also hitherto been restricted mainly to the use of aromatic polyisocyanates, because of the chemical inertness of aliphatic polyisocyanates. This, of course, cannot result in foams which are resistant to light and yellowing.
The specific examples of foam plastics with a top layer described in U.S. Pat. No. 3,847,720 have the same disadvantage since the polyisocyanates used for producing the top layer in the examples are aromatic polyisocyanates. Replacing the aromatic polyisocyanates in the formulations given in the examples of U.S. Pat. No. 3,847,720 with aliphatic polyisocyanates which react to produce light-fast polyurethanes would have the disadvantage that the systems could not be hardened within the necessary time of from 10 to 20 minutes at room temperature, even in the presence of catalysts.
U.S. Pat. No. 3,475,266 again gives no indication as to how molded foam plastics with light resistant and yellowing resistant top layers could be produced since the polyisocyanates used in this patent are again exclusively aromatic. In spite of the fact that these aromatic polyisocyanates are used, which are more highly reactive than aliphatic polyisocyanates, the process described is complicated by the fact that both the components and the mold have to be heated in order to achieve sufficient hardening.
It was therefore an object of the present invention to provide a simple and commercially practical process for the production of molded foam plastics based on polyisocyanates with a top layer which is resistant to light and yellowing. This problem could be solved by the process according to the invention which is described in more detail below.